1. Field of the Invention
The present invention relates to a technique of decision feedback equalization mainly utilized in a field of digital mobile communication.
2. Description of the Background Art
Recently, there has been progress toward successful application of digital communication techniques to various communication modes to be utilized on a moving object such as an automobile, a ship, or an airplane, which includes mobile communication with a fixed communication station or a communication satellite, TV broadcast reception, radio broadcast reception, or mobile communication for self-position tracking. Among the digital communication techniques utilized for this purpose, one of the most popular techniques is a so-called "Time Division Multiple Access" (abbreviated hereafter as TDMA) technique.
The TDMA technique is a technique for realizing a communication mode in which a plurality of stations transmit signals to a single relay station at the same passband, without timewise overlapping. In this technique, a prescribed constant period of time called a TDMA frame is established, and communication is performed within an allocated time slot in this TDMA frame. Thus, each station transmits bursty signals into the allocated time slot in the TDMA frame, under burst synchronization control to avoid interferences with the signals from other stations.
This type of communication system, which utilizes a radio transmission, usually involves a multipath, i.e., a multiple number of paths through which the radio signals can be transmitted between a transmitting station and a receiving station. The presence of the multipath creates a problem of severe multipath distortion on the signals.
In general, in a radio transmission involving the multipath, the transmission characteristic most often becomes as shown in FIG. 1 in which amplitude decreases as time elapses, which is a type of impulse response called a minimum phase mode, and where number of peaks shows the multipath reflections. On the other hand, there is another type of impulse response called a non-minimum phase mode in which amplitude increases as time elapses as shown in FIG. 2, although this mode appears far less frequently than the minimum phase mode does. It is known that the transmission characteristics of the minimum phase mode and the non-minimum phase mode are continually changing. Also, it is known that an average value E(d) of a delay spread d for the minimum phase mode and an average value E(d') of a delay spread d' for the non-minimum phase mode generally satisfy a relationship E(d)&gt;E(d').
In order to cope with the problem of distortion due to multipath transmission, a so-called decision feedback equalizer is utilized.
An example of a conventional decision feedback equalizer is shown in FIG. 3.
This decision feedback equalizer comprises a plurality of feedback taps 6 (6-1, 6-2, 6-3, and 6-4), a plurality of feedforward taps 7 (7-1, 7-2, 7-3, 7-4, and 7-5), an adder 8, a substractor 9 and a decision device 11. The feedforward taps 7 provide a forward part of the equalizer while the feedback taps 6 provide a feedback part of the equalizer, the signals from which are added together by the adder 8 to reconstruct the transmitted signal without the multipath distortion. The decision device 11 determines the binary values of the transmitted signal, i.e., which portion is 0 and which portion is 1 in the transmitted signal, according to the output of the adder 8. The output of the decision device 11 is fed back to the feedback taps 6. Meanwhile, the substractor 9 subtracts the output of the adder 8 from the output of the decision device 11 to obtain a difference signal e(t), so as to assess the appropriateness of tap coefficients given to the feedforward taps 7 and the feedback taps 6. The tap coefficient of each of the feedforward taps 7 and the feedback taps 6 are adjusted according to this difference signal e(t), as indicated by arrows 10. Further detailed description of the decision feedback equalizer can be found in "Adaptive Equalization", S. U. H. Qureshi, Proceeding of the IEEE, Vol. 73, No. 9, pp. 1349-1987, September, 1985.
The multipath distortion shown in FIGS. 1 and 2 can be removed by this decision feedback equalizer of FIG. 3 as follows.
In the case of the minimum phase mode of FIG. 1, the feedback taps 6 play a dominant role. In this case, by using the decision result for the multipath reflection peak at a time t=t.sub.A in FIG. 1, the multipath reflection peaks arriving at later times t=t.sub.B, t=t.sub.C, t=t.sub.D, t=t.sub.E, and t=t.sub.F are cancelled out.
On the other hand, in the case of the non-minimum phase mode of FIG. 2, the feedforward taps 7 play a dominant role. In this case, the signals received at the time t=t.sub.C', t=t.sub.B', and t=t.sub.A' are linearly synthesized to cancel out the multipath components, before making a decision for the signal at the time t=t.sub.E', and then the decision is made for the time t=t.sub.A'.
In general, the equalization of the non-minimum phase mode requires a considerably greater number of taps than the equalization of the minimum phase mode does. This is particularly true in a case in which the signals received at the time t=t.sub.A' and t=t.sub.B' (or at the time t=t.sub.C', or else at the time t=t.sub.E') have almost the same amplitude.
Thus, the conventional decision feedback equalizer requires a sufficient number of feedback taps to equalize the minimum phase mode, and a sufficient number of feedforward taps to equalize the non-minimum phase mode. Also, in order to satisfy the relationship E(d)&gt;E(d'), a considerable number of feedback taps has been required.
However, the increase of the number of feedback and feedforward taps considerably reduces the response speed of the equalizer with respect to the change of the transmission medium distortion, so that it has been impossible to utilize such an equalizer with many taps in mobile communication in which the transmission characteristic varies very rapidly.
Now, as shown in FIG. 4, in the TDMA technique, a training signal 18 is provided in a middle of bursty signal data 17, where the training signal 18 is sandwiched between the first transmission data 17-1 and the second transmission data 17-2. This position of the training signal 18 is used in order to secure a similar transmission characteristic at a time of transmission data reception and at a time of training signal reception.
In equalizing such a bursty signal data 17, the received signals are temporarily stored in a memory device first. Then, in order to obtain the second transmission data 17-2, the data are equalized as they are read out, starting from C toward D (in a normal time direction), from the memory device. On the other hand, in order to obtain the first transmission data 17-1, the data are equalized as they are read out, starting from A toward B (in a reversed time direction), from the memory device. Thus, the training signal 18 is indispensable in equalizing the received signals.
However, the equalization of such bursty signal data by the decision feedback equalizer has the following problem.
Namely, in equalizing the data starting from A toward B, the direction of time appears to be reversed, so that instead of dealing with the transmission characteristics of FIGS. 1 and 2, the equalizer has to deal with the mirror-image transmission characteristics of FIGS. 5 and 6. Thus, when the equalizer is designed to have only a sufficient number of a predetermined number of feedforward and feedback taps to deal with the transmission characteristics of FIGS. 1 and 2, it is not capable of dealing with the transmission characteristics of FIGS. 5 and 6.
Similarly, as shown in FIG. 7, the training signals 18-1 and 18-2 can be provided at ends of the bursty signal data 17', where the first transmission data 17-3 and the second transmission data 17-4 are sandwiched between the first training signal 18-1 and the second training signal 18-2.
In equalizing such bursty signal data 17', the received signals are temporarily stored in a memory device first as before. Then, in order to obtain the first transmission data 17-3, the data are equalized as they are read out, starting from E toward F (in a normal time direction), whereas in order to obtain the second transmission data 17-4, the data are equalized as they are read out, starting from G toward F (in a reversed time direction), from the memory device.
Here, again, in equalizing the data starting from G toward F, the direction of time appears to be reversed, and the same problem as described above occurs for this case as well.